Ion implantation has become a standard accepted technology of industry to dope workpieces such as silicon wafers or glass substrates with impurities in the large scale manufacture of items such as integrated circuits and flat panel displays. Conventional ion implantation systems include an ion source that ionizes a desired dopant element which is then accelerated to form an ion beam of prescribed energy. The ion beam is directed at the surface of the workpiece to implant the workpiece with the dopant element. The energetic ions of the ion beam penetrate the surface of the workpiece so that they are embedded into the crystalline lattice of the workpiece material to form a region of desired conductivity. The implantation process is typically performed in a high vacuum process chamber which prevents dispersion of the ion beam by collisions with residual gas molecules and which minimizes the risk of contamination of the workpiece by airborne particulates.
Ionized plasma is generated in a typical ion implanter in at least two separate locations. First, at the front end of an ion implanter, an ion source generates a plasma, from which an ion beam may be extracted, by ionizing an inert gas. An example of such an ion source is shown in U.S. Pat. No. 5,497,006 to Sferlazzo, et al., assigned to the assignee of the present invention and incorporated by reference as if fully set forth herein.
A simplified diagram of an ion source is shown in FIG. 1. A gas such as boron or phosphorous is input into an arc chamber AC via an inlet I and exposed to an energized filament F. The filament emits high-energy electrons E which are repelled by repeller R to confine the electrons to an ionization region between the filament and the repeller. The deflected electrons E collide with ionizable gas molecules in the ionization region, where the probability of collision with ionizable gas molecules is maximized. In this manner, a plasma is created comprised at least partially of positively charged ions. A generally positively charged ion beam is drawn from this plasma, typically through a source aperture SA in the arc chamber.
In addition to the repeller, a typical ion source also includes source magnets, as shown in FIG. 1 (power supplies not shown). The source magnets SM create a magnetic field across the arc chamber AC. The magnetic field alters the spiral path P of the electrons E emitted by the filament F and traveling through the arc chamber, in a well known manner, thereby increasing the probability of collisions with the ionizable gas molecules provided through inlet I and confined between the filament F and the repeller R. The source magnet SM current is adjusted to maximize ion beam current and beam quality. Accordingly, the source magnets SM and the repeller R confine the high-energy electrons emitted by the filament to the ionization region.
Also, a plasma is generated downstream in the implanter in a plasma shower. The plasma shower serves to counter the effects of wafer charging that the positively charged ion beam would otherwise have on a wafer being implanted. Such a system is shown in U.S. Pat. No. 4,804,837 to Farley, assigned to the assignee of the present invention and incorporated by reference as if fully set forth herein.
A simplified diagram of a typical plasma shower is shown in FIG. 2. The plasma shower comprises an arc chamber AC into which an inert gas such as argon is input via inlet I and exposed to an energized filament F. The filament emits high-energy electrons E that ionize the inert gas molecules to create a plasma within the arc chamber. The plasma diffuses through aperture A into the path of ion beam B passing through vacuum chamber VC. The plasma aids in neutralizing the net charge of the beam which in turn reduces the positive charge accumulation on the wafer as the ion beam strikes the wafer surface
The use of a repeller and a source magnet in an ion source, however, results in added complexity, cost, size, and power consumption of these devices. Further, source magnets create electrical noise that can perturb the plasma within the ion source. In addition, filaments in known plasma showers do not produce plasmas of sufficiently high density due to the lack of a containment mechanism for the high-energy electrons E emitted by the filament F. Moreover, attempts at increasing the plasma density typically require that the filament F consume significant amounts of energy.
Accordingly, it is an object of the present invention to provide a filament for use in a plasma generation source in an ion implanter such as an ion source or a plasma shower, which provides a low-noise, high density plasma while overcoming the deficiencies of known ion or plasma generation sources. It is a further object of the invention to provide a simple, energy efficient, economical and compact mechanism for primary electron confinement in an ion source or plasma shower to create a high density, low-noise plasma.